1. Field of the Invention
The present invention relates to dielectric ceramic compositions for use as resonators employed in a microwave frequency band of several gigahertz.
2. Description of the Prior Art
In recent years, attempts to use a dielectric material for a resonator or a filter used in satellite communication, broadcasting and microwave remote communication using a microwave having a frequency of several gigahertz or a transmitter-receiver such as a microwave remote recognition system.
In a resonator using such a dielectric material, the wavelength is decreased to 1/.epsilon..sup.178 (where .epsilon. is the dielectric constant) in the dielectric body. Accordingly, the larger the dielectric constant .epsilon. of the material is, the smaller the shape of the resonator can be. In addition, the loss of the material must be as low as possible so as to enhance the frequency selectivity and the stability. Furthermore, the temperature coefficient of resonance frequency .tau.f of the material is as close to zero as possible so as to increase the temperature setability.
Examples of this type of material conventionally known include materials of a BaO - TiO.sub.2 system, a Ba {(Zn.sub.1/3 (Nb.Ta) 2/3} O.sub.3 system, (Zr Sn) TiO.sub.4 system, and the like in terms of superior high frequency characteristics. However, the dielectric constants , of all the materials are low, i.e., 20 to 40.
On the other hand, materials having a high dielectric constant include a composition of a BaO - TiO.sub.2 - Nd.sub.2 O.sub.3 system which is proposed in, for example, Japanese Patent Laid-Open Gazette No. 8806/1986.
In this conventional ceramic composition, however, its dielectric constant , is high, i.e., 70 to 90, while its unloaded Q value which is the measure of low loss is small, i.e., approximately 1000 to 2000 in the neighborhood of 3 GHz.